1. Field of the Invention
The present invention relates generally to apparatus and methods used to mold composite materials to form composite structures. More particularly, the present invention is directed to the molds/tools that are used to form such composite structures.
2. Description of Related Art
Many processes for making composite structures utilize a mold or tool to provide desired surface contours and shapes. The mold is particularly important in autoclave processes where the uncured resin/fiber material is heated in the mold, under vacuum, to relatively high cure temperatures (350° F. and above) to form the final composite part or structure.
Molds made from steel alloys, such as INVAR36, are being presently used because they are extremely strong and can easily withstand the elevated cure temperatures used in autoclaves for composite material curing. These molds are also commonly referred to as “tools” or “tooling”. Steel alloy molds can be machined to obtain tight surface profile tolerances. In addition, steel alloy molds can be modified to provide different surface configuration and/or reconditioned by simply machining and polishing the mold surface. A further advantage is that steel alloy molds can be reused many times (in excess of 500 cycles) before they need to be reconditioned. Although steel alloy molds are well suited for their intended purpose, there are a number of drawbacks associated with the use of steel alloy molds. For example, steel alloy molds tend to be heavy and expensive. In addition, the time required to heat and then cool a massive steel alloy mold increases the cycle time during molding of composite structures.
Molds made from composite materials have been developed as an alternative to steel alloy molds. Composite molds have been popular because they are typically lighter and less expensive to make than steel alloy molds. The composite molds are generally formed using a highly accurate master mold that is made from a steel alloy or other suitable material. Examples of prior composite molds are set forth in U.S. Pat. No. 4,851,280.
Bismaleimide and polyimide resins have been used widely in combination with carbon fibers as the materials of choice for composite molds. Woven carbon fabric that has been hand cut into square pieces is applied to the master mold to form multiple individual layers. The pieces of woven fabric are oriented in the master mold to provide smooth tooling surfaces. Resin can be introduced into the woven fiber pieces in a number of ways. For example, the resin can be added through automated impregnation by the material manufacturer (prepreg). Alternatively, the resin can be introduced into the woven pieces when they are in the master mold. This is accomplished by vacuum infusion or simple hand application of the resin.
In general, a lower aerial weight woven fabric is used as the surface ply and/or surface resin gel coat for the composite mold in order to obtain a pit-free surface. Aerial weights for the surface fabric are usually on the order of 250 grams per square meter (gsm). The use of the low aerial weight woven fabric at the mold surfaces also tends to minimize the transfer or telegraphing of the relatively rough texture of underlying higher aerial weight woven fabrics (around 650 gsm) to the mold surface. The higher aerial weight fabrics are typically used in the body of the mold to build up the laminate bulk/thickness that is needed to achieve desired mold strength.
A problem with composite molds is that they are difficult to machine or repair. In addition, it is difficult to process the mold to obtain tight surface tolerances or accommodate changes in the profile of the mold surface without degradation of vacuum integrity and compromising dimensional stability. This problem arises because machining the surface of the mold exposes the heavier weight woven material and changes the quasi-isotropic characteristics of the laminate. The removal of layers of different weight material also leaves an unbalanced laminate that can distort and affect the dimensional integrity of the mold and compromise vacuum integrity. In addition, the heavier weight materials tend to be porous due to the woven characteristics of the material. When machining into these layers, the porosity affects vacuum integrity and vacuum leaks can develop along the interlaminar shear planes that are formed due to the material being applied to the master mold as layers. Machining of composite molds is further complicated because the resins used in composite molds tend to be brittle.